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On the Helicity of Oligomeric Formaldehyde.

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the crystals of 4, in which the neutral complexes are not linked
to one another by hydrogen bonds or even Na' ions, the observed v(NO) frequency is observed at 1515cm-'. I t thus appears at a much lower energy than in 2 (1719 cm-I) or in 3
(1669, 1642 cm-I). This effect must be attributed to the different strengths of the (p, + d,) donor bonds in the complexes: the
two Br- ligands in 2 are weaker n-donor ligands than the two
HO- ligands in 3, whereas the terminal 0x0 group in 4 is a very
strong n donor. The occupation of the n*(NO) orbitals increases
in the order Br- < HO- < 02-.which is related to a weakening of the N - 0 bond.
As described above, in the crystals of 5 the NO ligands are
coordinated to the Na' ions in two ways. The IR spectrum
therefore shows two v(NO) vibrations of relatively low intensity. one at 1529 cm- and the other at the very low frequency of
1476 cm- I ( ! ) . The corresponding 0x0 ligands are also coordinated in different ways: because of the coordination of 0 ( 3 ) to
Na(1) and its participation in an intramolecular 0 - H . . . O
bond. the Mo(1)-0(3) distance is 1.781(3) 8, and thus significantly longer than the Mo(2)-0(4) distance of 1.754(3) 8, where
0(4) only participates in a weak intermolecular 0 - H . . ' 0 bond.
The 0x0 group Mo(l)=O(3) in 5 is therefore a comparatively
weaker n donor, which is why the higher energy NO stretching
frequency of 1529 cm- should be assigned to this neutral complex. The N-O bond in the complex containing the
Mo(2)=0(4) group (stronger n donor) is weakened to a greater
extent. This effect may even be intensified by coordination of the
NO ligands to fwo Na' ions through the oxygen atom O(6). The
low-energy v(NO) stretching frequency of 1476 cm-' is therefore assigned to the neutral complex containing the (,u3-(NO)-tK N :2.3-dO) bridge. Although this interpretation is supported
by the two different N-O bond lengths of 1.241(5) and
1.265(6) A, the difference is just within experimental error (3 B ) .
Complexes 2, 3, 4, and 5 are isoelectronic (Mo-NOS4 compounds. The electron density on the molybdenum center is formally increased by the strong n donor
0'- in 4 and 5 but can again be lowered
by occupation of the rr*(NO) orbitals to
give a nitrosyl ligand of typical NOcharacter. This synergistic bonding
model is shown in Figure 5 : the d,, orbital localized on the central atom is occupied by two electrons in the (MoNO).4complexes as is the p orbital on
the 0'- ligand. whereas the n* orbital
on the NO ligand is initially unoccuFig. 5. Model of the
pied, which implies that n electron denin
sity is transferred from the 0'- ligand
to the Mo center and from there to the
and 5
n*(NO) orbital.
E.uprinientul Procedure
1: 36% HCI (10 mL) was slowly added to a stirred mixture of [MoL(CO),] (1.0 g.
2.3 mmol) and NaNO, (0.30 g, 4.3 mmol) in methanol (100 mL) to give a clear
yellow solution accompanied by foaming. After removal of the solvent. the yellow
residue was diasolved in H,O (200 mL), the resalting solution filtered. and a solution of K P F , (0.30 g) i n H,O (100 mL) added to the filtrate. A yellow precipirate
formed. Yield. 1.20g (90%).
2: Bromine (1.0 mL) u a s added to a solution of 1 (1.0 g. 1.7 mmol) in 48% HBr
(30 inL), which was then stirred for 10 min. The orange. microcrystalline product
obtained was filtered off and washed with ether. Yield: 1.0 g (75%). The analogous
PF; salt was obrained from 2 after addition of NaPF,
3. 3 a : A solution of 2 (1.0 g. 1.3 mmol) in H,O (301111) and tetrahydrofuran
(10 mL) was heated at reflux for 5 h. KPF, (1 .0 g) was added to the filtered solution.
Slow evaporation or the solvent cave red crystals of the monohydrate3a Yield:
0.60 g ( 8 5 % ) . The anhydrous compound 3 was obtained by recrystallization of 3 a
from CH,CI,
, VCH ~~rluh.sjirsellschufi
inbH, D-69451 W~.inhrim,1YY4
4: Solid NaOH (0.05 g) was added to a solution o f 3 (0.10 g. 0.17 mmol) in acetone
(1.0 mL). followed by stirring. After the color had changed from red to blue-green.
[Bu,N]Br (0.05 g) was added, and the solution was overlaid with n-hexane (1 mL).
Slow diffusion of the two layers produced crystals of 4. Yield: about 0.03 g.
5 : When synthesis of 4 was carried out without the addition of [Bu,N]Br. violet
crystals of 5 precipitated. Yield: about 0.03 g.
All compounds gave corrcct elemental analyscs (C.H.N)
Received: January 27,1994
Revised version: February 19. 1994 [Z 6647 I€]
German version: A n g e ~ Cliem.
1994, 106, 1556
[l] G. B. Richter-Addo. P. Legzdins. M c r u l Nirros,~/x.Oxford University Press,
New York, 1992.
[ 2 ] H. Adams. N. A. Bailey. G . Denti. J A. McCleverty. J. M . A. Smith. A. Wlodarczyk. J. C h m . S O ( .Dulron Trans. 1983, 2287.
[3] D. Sellmann. B. Seubert. F. Knoch, M. Moll, Z..4norr. A/%. Chfm. 1991. 600.
[4] J. H. Enemark. R D. Feltham. Coord. Chmr. Rev. 1974. 13. 339. According io
this reference, the electron configuration of B nitrosyl complex is given in the
[M-NO;" notation, where n is the number of electrons in metal d orbitals plus
the number of electrons in the n*(NO) orbital. or. in more simple terms. the
usual number o f d electrons of the L,M fragment if the NO is formally considered to he bonded as NO'. Jones. J. A. McCleverty. J Chzm. Sor Dalfon Em,.
1990. 3577.
[6] G. Haselhorst. S. Stoetzel. A. Strasshurger, W. W;ilc. K. Wieghardt, R. Nuber.
J. Cheni. Soc. Uulron Trans. 1993, 83.
[7] 3: C,,H,,N,O,MoPF,. crystals froin CH,CI,; monoclinic. space group P2,:c.
a =12.283(5). h = 8.237(3). c = 22.20(1)A, /j = 91.15(4)". Z = 4; /jrdlrd =
=00.73mm-1:48690bserved reflections(/> 2.Sr~(/)):empirical absorptions correction. Y scans. 2H,,,,, = 60"; R = 0.059; R, = 0.054,
max. residual electron density 0.48 e- k 3PF;
: IS disordered. C H in calculated
positions. O H not located on last difference Fourier map. 4: C,,H,,N,O,Mo,
crystals from acetone. monoclinic, space group Cc. u =13.624(3), h =
16.831(4).c = X.594(1) A. /I =107.1(1)'. Z = 4, prdlrd=1.46gcm-', L I , , ~ =
0 716 m m - ' ; 1404 obuerved reflections ( I 2 I O U ( / ) ) ;no absorption correction;
?Om,, = 50", R = 0.040. R, = 0.034: may. residual electron density
0.62 e - k'.C H in calculated positions. O H position obtained from difference
Fourier synthesis. The polarity was tested by carrying out refinement calculations in both enantiomeric configurations. The configuration giving smaller
R values was assumed to he the correct one. 5 : C,oH,,F,Mo,N,NaO,P; crystals from acetone monoclinic. space group P2,:[ (No 14). u = 9.809(2). h =
28.728(7), c = 15.144(4) A. p = 95.34(2)". 2 = 4, pLdlrd
= 1.62 gcin-'.
= ,0.716
m m - ' ; 5505 observed reflections ( I 2 2.Oa(I));no absorption correction. 2U,,,, = SO ': R = 0.045: R, = 0.042. mdx. residual electron denyity of
2.07 e - k' in the vicinity of the slightly disordered PF; aniona. C H in calculated positions. O H positions obtained from difference Fourier synthesis. The
attempts to model (split atom model) the disorder of the PF; anions were
unsuccessful; the intensities of the reflections were measured on a Syntex R 3 ( 3 )
01- on a Siemens P4 (4. 5 ) diffrdctometer at room temperature using Ma,
radiation (L = 0.71073 A). The structures were solved by Patterson syntheses (3)
or direct methods (4. 5 ) and refined for all independently observed reflections
using F (heavy atoms calculated with anisotropic and H atoms with isotropic
temperature factors); R, = { Z I C ( [~ l~~ l ) z J F J 2 ~Program
SHELTXL-PLUS (PC version: G. M. Sheldrick, Universitit Gottingen). Further details of the crystal structure investigations are available on request from
the Fachinformationsrentrum Karlsruhe. D-76344 Eggenstein-Leopoldshafen
( F R G ) . on quoting the depository number CSD-58173.
On the Helicity of Oligomeric Formaldehyde
Christian R. Noe," Christian Miculka, and Jan W. Bats
Since the formulation of the term "stereoelectronic factor" by
E. J. Corey a multitude of work has been carried out illustrating
the existence and the significance of this effect."] Nevertheless,
Prof. C. R. Noe. Dr. C. Miculka
Christian Doppler Laboratorium
Institut fur PharmaLeutische Chemie der Universitgt
Marie-Curie-Strdsse 9, D-60439 Frankfurt:Main (FRG)
Telefax : Int. code + (6Y)5800-9352
Dr. J. W. Bats
Institut fur Organische Chemie der Universitit Frankfurt
fJ57fJ-0833i94;1414-1476 3 /O.O(J+ .25:0
Angew. Cliem. In!. Ed. Engl. 1994. 33, No. 14
The main reason for
this concept still meets with
this is that most of the investigations are carried out on polar,
multifunctional carbohydrates, in which the relatively weak
stereoelectronic effects can often be hidden by other factors. In
addition, certain cases of the stereoelectronic effect, such as the
anomeric or guuclzr effect, are frequently interpreted as isolated
phenomena of individual bonds, and the stereoelectronic .prop.
erties of the molecule as a whole are neglected. The purpose of
investigations into stereoelectronic effects should be to indicate
the possibilities and limitations of the interpretation based on
considerations of the relevant frontier orbital interactions such
as n-o*. n-a*, n-n*, a-o*,and o-rc*, which are offered by
the very visual valence bond model.
Poly(oxymethy1ene) can be considered an almost ideal example of a compound with a stereoelectronically determined
conformation. because in this simple molecule each bond can be
fixed stereoelectronically. The helical conformation of polymeric formaldehyde has been known for a long time[31and clearly
differentiates this compound from polyethylene, which lacks
special stabilization and prefers the expected achiral unticonformation on steric grounds. Previously, we interpreted the gauche
configuration of oligomeric formaldehyde determined by X-ray
data‘“’ as an indication of the stereoelectronic effect: we noticed
a diastereoselective 1 . I 0 induction between inducing and
prochiral centers, which were separated by a tetra(oxymethylene) chain. The direction and extent of the observed
induction in the reaction was attributed to the helical stabilization of the chain by stereochemical effects. An X-ray structure
analysis of oligo- o r poly(oxymethy1ene)s providing structural
details such as bond lengths, bond angles, and torsion angles has
not yet been reported.
The melting point of the dimethyl-substituted pentamer of
formaldehyde is 18.3 T.[”
We attempted, therefore, to obtain
higher melting formaldehyde oligomers by simply heating
paraformaldehyde and benzyl alcohol under acid catalysis. We
chose benzyl alcohol since the phenyl group, as well as the
oxygen atom. is a “stereoelectronically active”, planar (pl) ligand[61on account of its o* orbital. This reaction provided a
mixture of chromatographically separable products 1 -5.”]
Crystals of the pentamer 5 suitable for X-ray analysis could
be obtained by low-temperature crystallization from chloroform/hexane.[*l The unit cell contains two independent molecules, which have a C, axis passing through the central carbon
atom (Fig. 1 ) . All the torsion angles in the chain are between
- 62” and - 68’ and indicate the expected gauche configuration,
which results in helicity. Since both helices have the same handedness and thus the same absolute configuration, it can be assumed that 5 crystallizes in two enantiomorphic crystal forms.
Consideration of the molecules in the unit cell indicates that
they are not identical but rather conformational isomers differentiated by the orientation of the benzyl groups. In molecule A
the continuous ( -)-guuc/ze conformation of the chain is continued up to C,, of the phenyl group; the aryl ligand thus assumes
the expected position of a pl ligand following the critera of the
b(ulky). pl(anar). H rule (Fig. 2).I6l In molecule B C,, of the
phenyl group lies almost antiperiplanar to the first carbon atom
Anjien. Clicm. lnt. Ed. Eng/ 1994. 33, No. 14
Fig. 1. Crystal structure of5: helical conformers with the same absolute conligurotion within the unit cell are shown.
Fig. 2. The two conformational isomers of5. In molecule A a nonbonding electron
pair of the first oxygen atom lies synperiplanar to the antibonding orbital of the
aromatic unit (PI ligand).
in the chain. thus assuming the preferred position of a b ligand
according to the b, pl, H classification.
The definition of “stereoelectronically determined” bond
lengths and angles is provided best by an oligomeric
oxymethylene structure. Uniform n-a* interactions between all
atoms along the chain occur without disturbance from other
structural elements. The concomitant regular shortening of
bonds should in turn be apparent in an X-ray structural analysis. In this respect the results are remarkable: there is not a
continuous, symmetrical reduction in bond lengths such that the
bonds are close to the same length. Rather a systematic pattern
of bonding is seen along the chain (Table I ) . In the middle of the
chain two shortened bonds (bond lengths ca. 139 pm) alternate
with two normal bonds (bond lengths ca. 142 pm). Each oxygen
atom has one shortened bond and one of normal length. Partic-
Table 1.
Observed bond lengths in the chain of 5 in pm (in each case the second half
of the molecule is ignored).
Molecule A
Molecule B
147.2(4) [a]
I 39.1 (4)
-~ 3.0
+ 2.9
[a] Values are given in boldface to emphasize the pairs of similar bond lengths
6’ VCH V ~ r l a g s g e s e l l ~ c hm~b~H~, f0-69451
Wemlwitn, 1Y94
0570-0833:94,’1414-1477 X 10.00+.25,0
ularly amazing is that the molecules A and B in the unit cell are,
to our knowledge, isomers of a type not previously known. For
each bond in the chain the difference in the bond lengths in the
two isomers is at least 1.7 pni (in the middle of the chain on
average 2.8 pm).
In the benzyl groups of conformer B the C,,-CH, bond is
shortened. This effect is made plausible by the almost orthogonal position of the aromatic unit to the C-0 bond and the
concomitant n - ~ *interactions. As is indicated by the bond
angle at the benzylic carbon atom (Fig. 3), this should be sub113.9"
The torsion angles in the middle of the chain (Table 2) indicate the deviation from the ideal staggered gauche configuration
because of the stereoelectronic effect, which leads to a slight
twisting of the poly(oxymethy1ene) helix (Fig. 4).
Fig. 4. Comparison of the secondary helices of molecules A and B. The smaller
variation in torsion angles in molecule A lead to a more regular secondary helix.
Fig 3. Patterns ofbond lengths and angles in the pentameric formaldehyde derivative 5 .
stantially weaker than the n - c * interactions in molecule A. It is
probable that the pattern of bond lengths in the oxymethylene
chain in A is influenced by the ends of the molecule.
I n connection with stereoelectronic effects the discussion often circles round the question of whether the geometry at the
oxygen atom is better described by canonical orbitals or, on
practical grounds, by the preferred, graphic model of sp' hybridization.""] Actually the observed angles lie consistently in a
range between 120" (trigonal planar) and 109.5" (sp' tetrahedral
angle). The oxymethylene structure offers the opportunity to
define a standard value for a "stereoelectronically determined"
bond angle at oxygen of 114.4' (Fig. 3).
While the bond angles at oxygen in both molecules A and B
indicate almost no variation, the carbon atoms in the chain have
an alternating pattern of bond angles. which should be eliminated by the stereoelectronic contribution to their bonding which
imparts an increased double bond character (which would be
characterized in turn by an increase in the bonding angle towards sp2).Thus. the 0 - C - 0 angle at each carbon atom between
two shortened bonds corresponds very closely to the angles at
the oxygen atoms (average value 114.7"); the other bond angles
are closer to the value at an sp3 atom (average value 11 1.3").
Tdble 2. Observed torsion angles i n the chain of5 in degrees ( i n each case the second
half of the molecule is ignored).
Molecule A
Molecule B
f l -C7-01-C8
- 68.1(0.3)
In connection with the e.yo anomeric effect, the @ torsion
angle allows us to define a standard value of 66.3" (cf. that
derived from the X-ray analysis of a glycolnitrile acetal[61 of
We are currently studying the structure of oligo- and poly(oxymethylene) helices and the asymmetric induction in their
Received: December 6. 1993 [Z6540IE]
German version: Angeic Cherii. 1994, 106, 1559
[ l ] a ) A. J. Kirby, Thr Anonreric Ejfecr ond Relutivl Stwwx4rcrronic Effects ut 0 . y ~
~ t v i Springer.
Berlin. 1983; b) P. Deslongchamps, Siereorkcrmnir E/jerrs in
Organic Clicmrsfr~( U r x . Cliern. S w h/.
I ) , Pergamon. Oxford. 1983.
[1] T.-C Wu. P. G. Goekjian. Y. Kishi. J Org. Chern. 1987, SZ. 4819-4823.
[3] a ) J. Hengstenberg. Anii. P h n . f h i p i g ) 1927, 84. 245-278: b) E. Sauter. Z .
Phrs. C h m . A h / . B 1932. 18. 417-435: c) M L . Huggins, J Cheni Phxs. 1945,
1.7. 37 42. d) C . F Hammer. T. A. Koch. J. F Whitncy. J '4.4. Po/)rn. Sri.
1959. 1. 169 17X: e) G. Cara~rolo.GK:. ('him. Irul. 1962, 92. 1345-1361. I ) G
Cararzolo. G. Putti. Chim. l i d . f . M f / U l l J 1963.45.771 -776: g) G . Cardnolo. M.
Mammi. J. P d w i . SCI.Par/ A Gen. Pup. 1963. 1. 965- 983; h) T. Uchida, H.
Takokoro. J Poijni. Sc,. P o / v n P / i ~ rE. d 1967, 5. 63- XI.
[4] C. R. Noe. M. Knollmuller. P. Ettmayer, Arigebi. Chcm 1988. /OO, 1431-1433:
Angrh. Cheni. / i l l . Ed. Et7gI. 1988. 27. 1379 -1381.
[S] R H. Boyd, J P o / j m . Sci. 1961. j0. 133 141.
[h] C . R. Noe, M. Knollmuller, G. Gostl. B. Oberhauser. H. Vollenkle, Axpea..
Chern. 1987. YY. 4 6 7 ~469; an fie^. Chem. Inr. Ed. En:/. 1987, 26, 442-444.
171 Experimental procedure: To a suspension of paraformaldehyde (12 g. 0.40 mol)
in benzene (60 mL) was added benryl alcohol (4.32 g, 0.04 mol) and sulfuric acid
(0.5 mL). and the mixture was heated at reflux for 2 h. The reaction mixture was
cooled and treated with solid sodium hydrogen carbonate ( 2 g). stirred for
30 min. and filtered. The reTidue was washed with ether and the filtrateconcentrated to drynehs. The crude mixture of oligomers (5.8 g) was separated by
columii chromatography (600 g fine silica gel, petroleum ether/ether. 15:I ) to
give1.60 g o f I . 0 . 9 0 g o f 2 . 0 . 6 0 g o f 3 . 0 . 2 0 g o f 4 . a n d 0 . 1 0 g o f 5 . Foranalytical
details x e C. Miculka. Dissertation. Technische Universitit. Vienna, 1993.
[8] Enraf-Nonius CAD4 diffractometer: room temperature, Cu,, radiation. ( I =
407h( I). h = 545.4(2). c = X41.6(3) pm. {I = 97.86(3)". V = IX53(2)x lo6 pm':
space group C2. Z = 4. p(Cukx)=7.3 cm-'. 20,,, = I 2 0 .; 2594 reflections, of
which 1496 were independent with I > 0 ; 227 parameters (C, 0 anisotropically
refined; position of the H atoms calculated); R = 0.039; R, = 0.030. Further
details of the crystal structure investigation can be obtained fron the Fachinformationventrum Karlsruhe. D-76344 Eggenstein-Leopoldshafen (FRG). on
quoting the deposition number CSD-57971.
- 66.4(0.3)
C'CH I.i~r/
rnhH, D-6Y4jl Wrtinheiiii, 1994
8 1/1.00+ .25:0
Angen. Chem. I n f Ed. Engl.
1994. 33. No. 14
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